Helical circular wave guides which transmit the TE.sub.01 mode constitute a very advantageous telecommunications trunk due to their low loss at decimetric and millimetric wavelengths. However, negotiating curves sets problems for these guides, since the TE.sub.01 mode then degenerates into another mode, Tm.sub.11 , which does not have the same properties so that the resulting loss greatly increases.
One means of reducing this loss in curves is constituted by helical wave guides which are particularly efficient when the guide includes a conductive screen outside the helix at a relatively short distance from the helix in relation to the wavelength.
One application of this principle had already been embodied by the use of enamelled copper wires wound in a helix on a mandrel and coated with thin layers of glass cloth tape and with a screen formed by wire gauze, metal tapes or metallized paper, all impregnated with polymerized expoide resin intended to harden the structure as a whole after possible reinforcing by other glass cloth or a mechanical protection tube and removal of the forming mandrel.
However, this type of guide had only relatively limited performance:
(a) it was possible to manufacture it only by a semi-continuous method which is slowed down by the time necessary for hardening the resin; and
(b) presently known polymerizable resins have a relatively wide dielectric loss angle such that tg .delta. is about 1 to 10.times.10.sup.-2, so that the curvature losses were not at all negligible, as shown by the formula established by G. COMTE and J. P. TREZEGUET (Cables et Transmissions No. 2, April, 1972, pages 166 to 182).
The application of this formula shows that for a dielectric thickness lying between the internal wall of the guide and the screen, the lower the loss factor K=.epsilon."/2.epsilon.' of the dielectric, the less the loss for a curve of radius R and it is seen also that the lower the permittivity .epsilon..sub.r ' of the dielectric the wider the guide's favourable frequency bank around curves (the lowest permeability is obtained when .epsilon..sub.r /.epsilon..sub.o =1.).
A first improvement had been made to this effect by forming continuously manufactured helical wave guides in accordance with French Pat. No. 1,604,891 of Dec. 31, 1968, invention of B. ORMILI and G. COMTE entitled "A machine for semi-continuous or continuous manufacture of helical circular wave guides" by means of an enamelled copper wire covered with adhesive tape intended to hold the wire and themselves covered with a conductive screen.
However, this realization has limits since it is quite a problem to hold the turns by adhesive tape and also the loss angle of these tapes cannot be very small, because of the type of material which constitutes the tapes (Mylar)(tg .delta. about 40.times.10.sup.-4) and the type of the adhesive which is generally polar and can have absorption maxima for millimetric waves.
Further, a low loss around curves can be maintained only if the instantaneous radius of curvature of the line does not produce sudden variations as would be the case if the mechanical protection of the guide did not sufficiently attenuate the effects of the stresses which are applied to it in particular during laying and due to the movements of the ground.
The helical circular wave guide remedies these disadvantages. It aims to produce minimal loss around curves (even curves of fairly small radius, e.g. 29m) for the widest possible frequency band, while having a very low cost price due to the continuous method used and to the choice of the materials which constitute it and providing sufficient mechanical protection for preventing sudden variations of the local radius of curvature of the guide.